Recipient is automatically added to outgoing email

I use the WLM 7 local client.  When I "reply all" to a message I receive from a certain sender, two specific people are automatically added from MY address book to the recipients of my reply. Those recipients automatically added to my reply were not included as recipients of the original message I received (either in the "To," "cc," or "bcc" parts of the message, nor were they listed under "Details" in the original message's "Properties."   I've explored "message rules" and other settings in WLM and Outlook, but have not unearthed any setting that causes this.  I'd appreciate having guidance to help solve this mystery.  Many thanks.